1. Field of the Invention
The present invention relates to an air-fuel ratio control system for an internal combustion engine, which is loaded thereon and has incorporated therein a fuel evaporative emission gas purge system for purging (releasing) an evaporated fuel adsorbed within a canister to a suction system of the internal combustion engine.
2. Description of the Related Art
In a fuel evaporative emission gas purge system, the fuel evaporative emission gas is adsorbed within a canister to prevent a fuel evaporative emission gas (HC) generated within a fuel tank from leaking into the atmospheric air. Also, a purge control valve is provided on a midway portion of a purge passage for purging the fuel evaporative emission gas adsorbed within the canister to a suction pipe of the internal combustion engine. By the use of this purge control valve, the amount of the fuel evaporative emission gas purged from the canister to the suction pipe is controlled.
Conventionally, as shown in Japanese Examined Patent Application Laid-Open Publication No. Hei 7-59917 or Japanese Patent Application Laid-Open No. Hei 7-293361, the amount of purge is controlled according to an air-fuel feedback correction factor FAF given as a control output of an air-fuel ratio feedback control (lambda control). Here, the air-fuel ratio feedback correction factor FAF is expressed by the following equation, using a calculated value of an amount of fuel injection TAU (hereinafter referred to simply as "the calculated TAU"). EQU FAF=calculated TAU/(TP.times.FTHA.times.FPA)-(FWL+FSE+FASE+FFC+FTC+FPRG+FLAF)
where TP: basic amount of injection (basic time period of injection)
FTHA: suction air temperature correction factor PA1 FPA: atmospheric pressure correction factor PA1 FWL: warm-up correction factor PA1 FSE: start-up time correction factor PA1 FASE: post-startup correction factor PA1 FFC: fuel cut restoration time correction factor PA1 FTC: acceleration/deceleration correction factor PA1 FPRG: purge correction factor, and PA1 FLAF: air-fuel ratio learning correction factor
It is to be noted that the above equation, which represents the FAF, can be determined by solving the following equation for the calculated TAU. EQU Calculated TAU=TP.times.FTHA.times.FPA.times.(FWL+FSE+FASE+FFC+FTC+FPRG+FLAF)
Meanwhile, with respect to the calculated TAU, a lower-limit guard value TAUmin and an upper-limit guard value TAUmax are set. Both values correspond to minimum/maximum amounts of injection of the fuel injection valve. When the calculated TAU falls outside the range of TAUman.ltoreq.calculated TAU.ltoreq.TAUmin, the calculated TAU is guard processed by the TAUmin or TAUmax, with the result that the calculated TAU=TAUmin or TAUmax. Accordingly, in the region wherein the calculated TAU is outside the range of from the guard value TAUmin to the guard value TAUmax, the amount of fuel injected from the fuel injection valve (hereinafter referred to simply as "the actual TAU") differs from the calculated TAU. For this reason, when control of the amount of purge is performed using the air-fuel ratio feedback correction factor FAF as in the prior art, the amount of purge control becomes an incorrect value that does not correspond to the actual TAU in the region wherein the calculated TAU is outside the range of from the guard value TAUmin to the guard value TAUmax. This makes it impossible to perform an accurate air-fuel ratio feedback control based on the consideration of the effect of purge, which results in deterioration of exhaust emission control.
In addition, in the second member of the right side of the above equation representing the FAF, air-fuel ratio fluctuating factors (the warm-up correction coefficient or factor FWL, start-up time correction factor FSE, post-startup correction factor FASE and fuel cut restoration time correction factor FFC) other than purge are included. Therefore, it is impossible to take out only an FAF fluctuated portion resulting solely from purge. Accordingly, when determining the amount of purge control according to the FAF as in the prior art, as other air-fuel ratio fluctuating factors than purge are contained also in the amount of purge control, it is impossible to perform accurate purge control. Therefore, it is impossible to perform an accurate air-fuel ratio feedback control based on the consideration of the effect of purge.